Strong <i>in Vitro</i> Cytotoxic Potential of New Ruthenium–Cymene Complexes

Two <i>p</i>-cymenerutheniumchlorido complexes with thiourea derivative of 7-chloroquinoline (<b>C1</b>) and pyridine-3-imidazole (<b>C2</b>) were synthesized starting from [(η⁶-<i>p</i>-cymene)­RuCl₂]₂ and corresponding ligands. The structures of complexes were determined with elemental analysis and IR, ESIMS, ¹H and ¹³C­{¹H} NMR, and 2D ¹H–¹⁵N correlation NMR spectroscopy. Cytotoxic activities examined by the MTT assay were performed in five human neoplastic cell lines (HeLa, K562, A549, MDA-MB-231, EA.hy926) and one nontumor human fetal lung fibroblast cell line (MRC-5). Tested complexes exhibited low micromolar activities with IC₅₀ in the range 11.03–56.45 μM, while ligands <b>L1</b> and <b>L2</b> were significantly less active. Complex <b>C1</b> showed cytoselective activity toward the K562 cell line (IC₅₀ = 11.03 ± 1.39 μM) and was 3 times less active against the nontumor MRC-5 cell line. Flow cytometry analysis indicated that complexes <b>C1</b> and <b>C2</b> after 24 h treatment caused a concentration-dependent increase of the apoptotic sub-G1 fraction (up to 18.4%), comparable to <i>cis-</i>diamminedichloridoplatinum­(II) (cisplatin, <b>CDDP</b>), although without other substantial alterations of the cell cycle. A drug-accumulation and DNA-binding study performed by ICP-MS in the K562 cell line revealed that complex <b>C1</b> had a high intracellular uptake (1.38 μg Ru/10⁶ cells), which significantly exceeded the intracellular uptake levels of <b>CDDP </b>(0.29 μg Pt/10⁶ cells) and <b>C2 </b>(0.08 μg Ru/10⁶ cells). However, both ruthenium complexes <b>C1</b> and <b>C2</b> bind to cellular DNA less efficiently in comparison to <b>CDDP</b>. The structure–activity relationship clearly suggested that introduction of a 7-chloroquinoline moiety in the ruthenium­(II)-<i>p</i>-cymene complex significantly contributed to the intracellular uptake of <b>C1</b> and higher cytotoxicity and cytoselectivity